Filters used in high-frequency applications, such as disk drive, video, and data transmission applications are generally implemented as continuous-time active filters. These continuous-time active filters are often implemented using transconductance-capacitor ("g.sub.m -C") filters. Continuous-time active filters of a desired order may be constructed by serially coupling or connecting g.sub.m -C filter stages until a filter having the desired order and response is provided. For example, continuous-time active filters configured as biquadratic filters may be provided using g.sub.m -C filter stages. A biquadratic filter is one whose transfer function contains complete quadratic equations in both the numerator and the denominator and can be implemented, for example, as a low pass filter, a high pass filter, or a notch filter.
In many applications, such as in data transmission and disk drive applications, it is advantageous to provide a continuous-time active filter with a constant group delay characteristic over a desired range of frequencies to prevent distortion of a signal waveform. The group delay may be defined as the negative of the derivative of the phase with respect to frequency. Furthermore, it is often advantageous and desirable to provide an adjustable or selectable group delay that provides a constant group delay at the selected group delay. It is also often advantageous and desirable to provide amplification of selected frequency components or frequency spectrums. The amplification of selected frequency components or frequency spectrums may be referred to as "boost."
For illustration purposes, the standard second order low pass filter transfer function is provided below: ##EQU1##
In order to provide an adjustable group delay that will be constant throughout the frequency spectrum, and boost, the standard second order low pass transfer function may be converted into the following equation: ##EQU2## where the term "-ks.sup.2 " is a boost term, and the term "bs" is an asymmetric zero term which is used to provide the adjustable group delay that is constant throughout the frequency spectrum. The boost term, -ks.sup.2, increases high frequency gain by adding two real symmetric zeros to the transfer function. Since one of the zeros is positive and the other is negative, the phase is not changed. The amplitude of the boost may be programmable and may be adjusted according to the value of "k." The asymmetric zero term, bs, causes the real zeros to no longer be symmetric which results in a change in the phase and hence the group delay of the transfer function. In order to provide asymmetry, b may be provided as either a positive value or a negative value, but not at a zero value. Thus, the group delay may be adjusted by changing the "b" term of the asymmetric zero term.
In disk drive applications, the boost may be used for such applications as pulse slimming and/or read channel equalization. The asymmetric zeros or adjustable group delay may be used in such applications to adjust the group delay characteristics of the disk drive read channel or data channel to optimize performance.
Prior attempts at providing boost and adjustable group delay have proven unsatisfactory at best. For example, one prior attempt at providing boost and adjustable group delay involved the use of an amplifier for amplifying an input signal to a g.sub.m -C filter stage and driving the bottom plate or electrode of the output capacitor of the g.sub.m -C filter stage. This presented serious technical problems due to the existence of a parasitic capacitor whose bottom plate could not be driven by the amplifier. In some cases, the value of the parasitic capacitor was up to thirty percent of the value of the output capacitor of the g.sub.m -C filter stage. As a result, the effective boost term, ks.sup.2, was greatly reduced. Because the boost term was significantly reduced, an amplifier having a very high gain had to be provided to overcome this limitation. The high gain amplifier significantly increased overall power consumption. Also, the value of the parasitic capacitance was difficult, if not impossible, to predict because of semiconductor fabrication variations. This required the use of an amplifier with adjustable gain which further increased circuitry and costs. Furthermore, the amplifier generally had a limited bandwidth which reduced its effectiveness for applications using high frequency signals.
Another prior attempt at providing boost involved amplifying a current provided through the capacitor of a g.sub.m -C filter stage of a continuous-time filter. This technique has also proven unsatisfactory. The current provided through the capacitor is often large and consumes a significant amount of power when amplified. The increased power consumption results in increased circuitry area to handle the increased power consumption. The increased circuitry area increases undesirable circuitry parasitics.